Conventionally, a method in which a sensor module having a configuration in FIG. 10A is used (for example, see Patent Document 1) and a method in which a sensor module having a configuration in FIG. 10B is used (for example, see Patent Document 2) are known as a method for detecting a heat flow flowing out from a body surface and measuring (calculating) a core body temperature from a detection result.
For use of one sensor module in FIG. 10A, namely, a heat flux sensor in which temperature sensor is attached to each of top and bottom faces of a heat insulator, a core body temperature Tb is calculated using the following equation (1) from a temperature Ta measured with the temperature sensor on the top side of the heat insulator and a temperature Tt measured with the temperature sensor on the bottom side of the heat insulator.Tb=(Tt−Ta)Rx/R1+Tt   (1)
Where R1 and Rx are a heat resistance of a heat insulator and a heat resistance of a subcutaneous tissue, respectively.
In the internal temperature calculating method in which the sensor module in FIG. 10A is used, basically fixed values are used as the heat resistances R1 and Rx. However, because the heat resistance Rx depends on a location or an individual, a measurement error is included in the core body temperature Tb, which is calculated from the equation (1) using the fixed value as the heat resistance Rx, according to a difference between the heat resistance Rx used and the actual heat resistance Rx. Therefore, sometimes time changes of the temperatures Tt and Ta are measured, and the heat resistance Rx is calculated from the measurement result (see Patent Document 1).
In the case that an internal temperature is calculated with a sensor module in FIG. 10B, a temperature difference expressing a heat flux from the body surface is measured with each of two heat flux sensors having different heat resistances of the heat insulators. The following two equations can be obtained when the temperature difference is measured with the two heat flux sensors having different heat resistances of the heat insulators.Tb=(Tt−Ta)Rx/R1+Tt   (2)Tb=(Tt′−Ta′)Rx/R2+Tt′  (3)
Where R1 and R2 (<R1) are heat resistances of heat insulators of the heat flux sensors as illustrated in FIG. 10B. Ta and Tt are temperatures measured with the temperature sensors on the top and bottom sides of the left heat flux sensor (a heat flux sensor in which a heat insulator having a heat resistance R1 is used) in FIG. 10B. Ta′ and Tt′ are temperatures measured with the temperature sensors on the top and bottom sides of the right heat flux sensor (a heat flux sensor in which a heat insulator having a heat resistance R2 smaller than the heat resistance R1 is used) in FIG. 10B.
When Rx is deleted by a combination of the equations (2) and (3), the following equation (4) can be obtained.
                    [                  Mathematical          ⁢                                          ⁢          formula          ⁢                                          ⁢          1                ]                                                            Tb        =                                            R              ⁢                                                          ⁢              2              ⁢                              (                                  Tt                  -                  Ta                                )                            ⁢                              Tt                ′                                      -                          R              ⁢                                                          ⁢              1              ⁢                              (                                                      Tt                    ′                                    -                                      Ta                    ′                                                  )                            ⁢              Tt                                                          R              ⁢                                                          ⁢              2              ⁢                              (                                  Tt                  -                  Ta                                )                                      -                          R              ⁢                                                          ⁢              1              ⁢                              (                                                      Tt                    ′                                    -                                      Ta                    ′                                                  )                                                                        (        4        )            
Accordingly, in the sensor module of FIG. 10B, the core body temperature Tb can be calculated without being affected by an individual difference of the heat resistance Rx of the subcutaneous tissue.